1. Field of the Invention
The present invention relates to a zoom lens, more particularly to a cam mechanism, incorporated in the zoom lens, for moving optical elements such as lens groups.
2. Description of the Related Art
In conventional zoom lenses, it is often the case that a single cam ring incorporated therein is provided on an inner peripheral surface or an outer peripheral surface of the cam ring with two sets of cam grooves for moving two linearly guided optical elements (a front optical element and a rear optical element), e.g., two lens groups, respectively. Miniaturization of this type of cam ring sometimes requires the two sets of cam grooves to be formed to intersect each other, and accordingly, measures need to be taken in a zoom lens having such a cam ring to prevent the associated two sets of cam followers that are respectively engaged in the two sets of cam grooves from being disengaged therefrom at points of intersection of the two sets of cam grooves. As an example of a mechanism for preventing the cam followers from being disengaged from the associated cam grooves, a mechanism which includes auxiliary partial cam grooves and associated sub-cam followers for preventing disengagement of the two sets of cam followers from the two sets of cam grooves is known in the art, wherein the auxiliary partial cam grooves are provided independently of the two sets of cam grooves (main cam grooves) for guiding the associated cam followers, and the sub-cam followers are made to be engaged in the auxiliary partial cam grooves when the two sets of cam followers pass through the aforementioned points of intersection of the two sets of cam grooves. This type of mechanism is disclosed in, e.g., Japanese unexamined patent publication H10-282394.
However, taking such measures for the prevention of disengagement of each cam follower is liable to complicate the cam mechanism and increase the size thereof. Additionally, if the aforementioned two sets of cam grooves are formed on the cam ring to be isolated from each other in the optical axis direction so as not to intersect each other, the length of the cam ring becomes great to thereby make it difficult to miniaturize the zoom lens.